Orinoco Guiana Shield

Geography & hydrology

The Orinoco Guiana Shield is freshwater ecoregion 308 in the Freshwater Ecoregions of the World scheme (Petry & Hales), one of a cluster of Orinoco ecoregions and classified as a tropical and subtropical upland-river system. It covers the lowland and highland country of the Guayana Shield that drains, by way of the Orinoco's right-bank tributaries, into the main river. Its limits are hydrological: the upper Orinoco–Río Negro divide to the south (excluding most of the Casiquiare and the Negro), the Río Branco divide to the east, the Cuyuní (Essequibo) divide to the north, and the mainstem of the Orinoco itself to the west. Politically it lies almost entirely in Venezuela, chiefly in Amazonas and Bolívar states (FEOW Ecoregion 308; Lasso, Machado-Allison & Taphorn, 2016).

The principal rivers are the upper Orinoco and a fan of large clearwater and blackwater tributaries: the Ventuari, Atabapo, Suapure, Cuchivero, Caura, Aro, Paragua and Caroní. These are not sluggish lowland rivers but upland ones, falling off an immense and ancient landscape — the remnants of an ancient sandstone tableland laid over a still older granitic basement, the Guayana (or Guiana) Shield. The terrain runs from forested lowlands and rounded hills to undulating high plains and the tepuis, sheer-walled sandstone table mountains whose summits reach roughly 2,2625 ft. It is on one of these, the Auyán-tepui in the Caroní drainage, that the Río Churún drops as Angel Falls (Kerepakupai Merú), at about 3212 ft the highest waterfall on Earth (FEOW Ecoregion 308).

Two features make the hydrology unusual. First is the abundance of rapids, cataracts and falls — the Spanish raudales — wherever a river crosses a resistant band of shield rock; the Caroní alone tumbles through major cataracts at Salto Pará and elsewhere before reaching the Orinoco at Ciudad Guayana. Second is the Casiquiare connection: along the ecoregion's southern edge the Orinoco sends a distributary, the Río Casiquiare, southwest into the Río Negro and thus into the Amazon, one of the few natural channels on the planet that links two major river basins. That bifurcation lets fishes move between the upper Orinoco and the Negro, and it is one reason the shield's faunas are shared across basin lines. The whole system runs on a single seasonal flood pulse — high water roughly May through August in the Venezuelan wet season, low and clear water in the dry months — which connects and then isolates channels, pools and flooded forest on an annual rhythm (FEOW Ecoregion 308; Lasso et al., 2016).

Water & habitat

In the limnological language Harald Sioli devised for Amazonian rivers, the Orinoco Guiana Shield is the basin's clear- and blackwater half — the antithesis of the silt-laden, near-neutral 'white water' that the Andean tributaries carry into the Orinoco. The shield is geologically ancient and deeply weathered, so its rivers run off rock that has already given up almost everything soluble; sediment loads are minuscule and the water is correspondingly dilute, soft and acidic (Lasso, Machado-Allison & Taphorn, 2016; FEOW Ecoregion 308).

Where a river drains forested shield with little leaf-litter staining, the result is a clearwater stream: transparent, green-tinted, low in dissolved ions, running over pale sand, water-polished granite and quartzite cobble. The Ventuari, largest tributary of the upper Orinoco, is the classic example — a clearwater river of sand banks and rocky rapids (Aquarium Glaser, on Crenicichla zebrina). Where the catchment is swampy or carpeted in white-sand and Mauritia palm stands (the morichales), tannins and humic acids leach in and the water turns blackwater: tea-brown, transparent in a wine-glass but light-absorbing in the channel, strongly acidic and almost devoid of dissolved minerals. The Atabapo, on the ecoregion's southwestern margin, is among the most celebrated blackwaters in South America. Detailed geochemistry from the Caura confirms the pattern at the basin scale: a dilute, organic-rich shield river whose dissolved iron and aluminium track its dissolved organic carbon, with chemical weathering and CO2-consumption rates that are high for the Orinoco yet still leave the water poor in the major cations that harden water elsewhere (Mora, Alfonso, Baquero, Balza & Pisapia, 2010).

Two habitat themes recur. One is the rapids: fast, shallow, oxygen-rich water over bare crystalline rock, riddled with crevices and seasonally clothed in river-weeds and benthic algae — a dense patchwork of microhabitats sorted by depth, flow and substrate that rheophilic (current-loving) fishes partition among themselves. The other is the still, warm, hypoxic backwater: lakes, ponds and forest pools in the savanna-and-shield mosaic that, especially in the dry season, run low in dissolved oxygen and acidic, conditions so demanding that the Guiana Shield holds an unusually high share of air-breathing and acid-tolerant fishes (de Souza, Armbruster & Willink, 2020). The Caura and most shield rivers record some of the lowest dissolved-oxygen levels known for river systems — a fact with a dark conservation twist, since low oxygen favours the bacterial conversion of mining mercury into its most toxic, bioaccumulating form (Mongabay, 2006).

The cichlid fauna

The Orinoco Guiana Shield is not a hyper-endemic hotspot in the way that single isolated rivers can be — FEOW puts species-level endemism below 10%, with 49 endemics of which more than 40% are armored loricariid catfishes and most of the rest characins. But cichlids are woven all through the fauna, and the genera here read like a survey of how perciform fishes have learned to live in dilute, rocky, seasonally pulsing shield water (FEOW Ecoregion 308). Two of the ecoregion's named cichlid endemics in fact belong to this group: the eartheaters Geophagus grammepareius and Geophagus taeniopareius, both described from the Orinoco's shield tributaries and effectively restricted to them (Kullander, Royero & Taphorn, 1992).

The Geophagus 'eartheaters' are the signature substrate-sifters of the region, taking mouthfuls of sand and rapids detritus and winnowing out invertebrates through the gills. The roster runs deep in them — Geophagus abalios, G. dicrozoster and G. winemilleri from the Orinoco–shield mosaic, the rapids-associated G. grammepareius and G. taeniopareius, plus G. gottwaldi from the upper Orinoco–Atabapo blackwaters. Alongside them sit their relatives in the same earth-eating guild: Satanoperca mapiritensis, a 'demonfish' eartheater of the basin, and the genus Guianacara, represented here by the shield-endemic Guianacara stergiosi. Sharing the slow margins are the larger acaras — Aequidens metae and the tepui-region Aequidens chimantanus, the smiling Bujurquina mariae, and the deep-bodied severum relatives Heros liberifer (a mouth-then-substrate brooder of the Orinoco) and the spectacular, blackwater-loving Uaru fernandezyepezi of the Atabapo and upper Orinoco, a triangular cichlid prized and rarely seen in the hobby.

The rapids themselves belong to the predators. Crenicichla, the pike cichlids, reach their most striking form here in Crenicichla zebrina, the zebra or 'firemouth' pike cichlid discovered in 2002 and described in 2008 from the clearwater rapids of the lower Ventuari — a large, boldly barred ambush hunter that was so hard to collect the original describers had only two specimens (Aquarium Glaser, 2011). Above them in the water column cruise the peacock basses: Cichla intermedia, the royal peacock bass, is endemic to the Orinoco and Casiquiare drainages and is a rocky-channel and rapids specialist, its slender, many-barred body built for the fast clear water of the shield. At the other end of the size scale are the dwarfs. Mikrogeophagus ramirezi — the ram, one of the most beloved aquarium cichlids in the world — is at home in the warm, soft, gently flowing waters of the Orinoco system, and the ecoregion is crowded with Apistogramma: A. hongsloi, A. velifera, A. megaptera, A. psammophila, A. piaroa, A. helkeri and a long tail of recently described, often micro-endemic relatives (A. caudomaculata, A. flabellicauda, A. inornata, A. intermedia, A. lineata, A. minima, A. pedunculata, A. velifera). These thumb-sized fishes hold tiny territories in the leaf litter and root tangles of blackwater creeks, and several are known from only a handful of localities — a reminder, FEOW notes, that the shield's headwaters remain only fairly to poorly explored and almost certainly hold undescribed forms (FEOW Ecoregion 308).

Conservation

The defining threat to the Orinoco Guiana Shield is gold. The ecoregion's headwater streams — particularly in Bolívar and Amazonas states — are being torn up by small-scale and increasingly large-scale mining, which strips banks, dumps suspended sediment into rivers built on the absence of it, and releases mercury used to amalgamate gold from the gravel (FEOW Ecoregion 308; Lasso, Machado-Allison & Taphorn, 2016). In 2016 the Venezuelan government formalized this with the Arco Minero del Orinoco (Orinoco Mining Arc), a mining concession zone of roughly 111,497 mi² carved out south of the Orinoco without prior environmental impact study or indigenous consultation — an area larger than many countries, overlapping the catchments of the Caura, Caroní and Cuchivero (Lewis & Clark Law / IELP, 2021; CSIS, 2022).

The damage is most starkly documented in the Caura, the basin's second-largest shield river and the source of a large share of the drinking water south of the Orinoco. Despite being declared a national park in 2017, the Caura has been overrun by thousands of illegal miners operating from its lower reaches up toward Salto Pará. Because the Caura and rivers like it carry some of the lowest dissolved-oxygen loads of any river system on Earth, conditions strongly favour bacterial methylation of mercury — the chemical step that turns spilled metallic mercury into methylmercury, the form that climbs the food chain through algae, invertebrates and fish into the people who eat them (Mongabay, 2006). A health survey found that the great majority of Ye'kwana and Sanima women along the Caura carried mercury burdens far above World Health Organization guidance, with a substantial fraction at levels posing neurological risk to their children (Caracas Chronicles, 2020). For the river's cichlids the same mining is doubly destructive: it both poisons the food web and chokes the clear, oxygen-rich, rock-floored rapids that Crenicichla, Cichla, Geophagus and the rapids-associated eartheaters depend on, filling the rocky interstices with silt and erasing the very microhabitat structure that sorts the fauna.

Beyond mining, the basin faces the standard pressures of a frontier — deforestation, wildfires, agricultural biocides and untreated sewage near growing towns, and overfishing of the migratory catfishes (Brachyplatystoma, Zungaro) that run from the lower Orinoco up into these shield tributaries to spawn (Lasso et al., 2016). The Caroní is additionally re-plumbed by the giant Guri (Simón Bolívar) hydroelectric dam, which long ago converted a long stretch of that rapids river into one of the world's largest reservoirs and altered downstream flow. The conservation paradox of the Orinoco Guiana Shield is that its remoteness and low human density once protected it — and that the same dilute, oxygen-poor, sediment-free water that makes it a refuge for specialized cichlids is exactly what makes it so quick to amplify mercury and so slow to recover once the rapids are silted and the food web is loaded with metal (FEOW Ecoregion 308; Mongabay, 2006; CSIS, 2022).

Sources

1. Orinoco Guiana Shield — Freshwater Ecoregions of the World (FEOW Ecoregion 308; boundaries, rivers, tepuis, endemic fishes, mining threat)
2. Lasso, C.A., Machado-Allison, A. & Taphorn, D.C. (2016). Fishes and aquatic habitats of the Orinoco River Basin: diversity and conservation. Journal of Fish Biology 89(1):174-191
3. Mora, A., Alfonso, J.A., Baquero, J.C., Balza, L. & Pisapia, D. (2010). Caura River basin: weathering rates, CO2 consumption, and chemistry of major and trace elements in an Orinoco tributary from the Precambrian Guayana Shield. Geochemistry, Geophysics, Geosystems 11(11)
4. de Souza, L.S., Armbruster, J.W. & Willink, P.W. (2020). Connectivity of Neotropical river basins in the central Guiana Shield based on fish distributions. Frontiers in Forests and Global Change 3:8 (Sioli water-chemistry classes; hypoxic savanna waters)
5. Vari, R.P. et al. — Checklist of the Freshwater Fishes of the Guiana Shield (Smithsonian, Bulletin of the Biological Society of Washington): water types and ichthyofauna of the shield
6. Mining in Venezuelan Amazon threatens biodiversity, indigenous people (Mongabay, 2006): Caura low dissolved oxygen and mercury methylation risk
7. Caura: a Maduro-made environmental disaster in the Amazon (Caracas Chronicles, 2020): illegal gold mining, mercury burdens in Ye'kwana/Sanima, national-park status
8. Orinoco's Mining Arc: an environmental crime with global effects (Lewis & Clark Law School, International Environmental Law Project, 2021): Arco Minero del Orinoco scale and legality
9. Illegal Mining in Venezuela: Death and Devastation in the Amazonas and Orinoco Regions (CSIS, 2022): Arco Minero, armed actors, mercury and environmental damage
10. First import! Crenicichla zebrina arrived (Aquarium Glaser / Frank Schäfer, 2011): zebra pike cichlid, lower Ventuari clearwater, discovery and import history